This invention relates to a protective relaying device which is adapted to protect an electric power system.
The protective relaying device is used to protect an electric power system by instantaneously isolating a faulty part of the system from other parts, thus preventing the faulty effect from extending to the other parts of the power system.
However, where any internal fault occurs in the protective relaying device, an erroneous operation, such as erroneous isolation of valid part or parts from another part of the power system, or making it to isolate the faulty part from another valid part tends to occur, thus causing serious effects on the operation of the power system.
For instance, when a fault occurs in a potential transformer connected to a bus bar or a transmission line for supplying a voltage to a protective relaying device, the output of the potentional transformer is lowered. The protective relaying device receiving the lowered voltage tends to misunderstand that the lowered voltage is caused by a fault in the power system, thus isolating a valid part of the power system from another part, or making it difficult to isolate the faulty part from another part because of its reduced input voltage.
A known device for preventing the difficulties of the conventional protective relaying device is illustrated in FIG. 1.
In this device, a potential device 2 comprising a first potential transformer PT.sub.1 and a second potential transformer PT.sub.2, is connected to a bus bar 1. A current transformer 4 is connected to a transmission line 3. A first relaying element 5 is provided to receive the output V.sub.1 of the first potential transformer PT.sub.1 of the potential device 2 as its input voltage and the output of the current transformer 4 as its input current. The first relaying element 5 thus judges the validity of the transmission line 3 from the magnitude and phase relation of these inputs, and operates to protect the line 3.
A second relaying element 6 is also provided to receive the same output from the first potential transformer PT.sub.1 and the current transformer 4, and to protect the same transmission line 3.
The first and second relaying elements 5 and 6 are ordinarily termed a faulty section selecting relay and a fault detecting relay, respectively. The first relaying element 5 has a narrower protection range than that of the second relaying element 6, and is responsive only to a fault in a section to be protected by the relaying element 5, while the second relaying element 6 includes, in its protecting range, the protection against the erroneous operation of the relaying element 5 caused by its internal fault.
In the above described conventional protecting relaying device, there is further provided a voltage-unbalance detecting relay 7 which receives the output voltages V.sub.1 and V.sub.2 of the first and second potential transformers PT.sub.1 and PT.sub.2 of the potential device 2, and constantly surveys these values to detect any unbalance therebetween. When an unbalance is detected, the detecting relay 7 delivers an output S.sub.3 of a logic "1" which is further inverted by a NOT circuit 8.
An AND gate circuit 9 is connected to receive the outputs S.sub.1 and S.sub.2 of the first and second relaying elements 5 and 6, and the output S.sub.4 of the NOT circuit 8 as its inputs. When these three inputs are applied, the AND gate circuit 9 is enabled to deliver a tripping instruction to a circuit breaker 12 in the transmission line 3.
The above described conventional protective relaying device operates as follows.
When the potential device 2 including two potential transformers PT.sub.1 and PT.sub.2 has no fault, the protection of the power transmission line 3 is carried out correctly by the output of the AND gate circuit 9 which delivers a tripping signal to a circuit breaker 12 when the three inputs S.sub.1, S.sub.2, and S.sub.4 are applied.
However, when a fault occurs in the circuit of the potential device 2, the outputs of the two potential transformers PT.sub.1 and PT.sub.2 are unbalanced, causing the input voltage of one of the relaying elements 5 and 6 to become lower than the other. The relaying element having a reduced input voltage misunderstands the reduction as a result of a fault in the transmission line 3, and tends to operate erroneously.
The voltage unbalance detecting relay 7 has been provided to eliminate this difficulty. When the output voltages V.sub.1 and V.sub.2 of the first and second potential transformers are different, that is, when a fault occurs in the circuit of the potential device 2, the output S.sub.3 of the relay 7 becomes "1". The output S.sub.3 is inverted by the NOT circuit 8 into "0", which is applied to an input of the AND circuit 9.
Accordingly, although a fault occurs in the potential device 2 and either one of the relaying elements 5 and 6 operates, the voltage unbalance detecting relay 7 prevents the delivery of any erroneous tripping signal from the AND gate circuit 9 to the circuit breaker 12.
In the known device shown in FIG. 1, even when a fault occurs in the potential device 2 and unequal voltages are supplied to the relaying elements 5 and 6, the voltage unbalance detecting relay 7 prevents the delivery of the erroneous tripping instruction to the circuit breaker 12. However, the provision of the relaying element 7 has not only increased the production cost, but also reduced the reliability of the conventional protective relaying device.